Botulinum neurotoxin (BoNT) is an extremely potent neuromuscular blocker that has been designated a category A select bioterror agent by the United States Government because of its extreme lethality and its availability from environmental sources. Current countermeasures for BoNT exposure are insufficient, and there is an urgent need for therapeutic antibodies able to counteract the effects of BoNT exposure. Our working hypothesis is that a combination of human antibodies that bind BoNT will potently protect individuals from the complications of BoNT exposure. We have recently developed a novel hybridoma protocol for the cloning of human antibodies and have used it to clone antibodies that bind BoNT serotypes A and B. One of the serotype A antibodies is neutralizing in vivo. We plan to use our method to clone native human antibodies that bind and neutralize botulinum toxin serotypes A and B. For this purpose we will obtain peripheral blood lymphocytes from individuals vaccinated with the botulinum toxoid vaccine. We will assess in vitro and in vivo neutralizing activities of the cloned antibodies, singly and in combination. We will characterize the binding and primary sequences of the antibodies to determine the features that correlate with the ability to neutralize toxin and to cooperate synergistically in toxin neutralization. The primary result of these experiments will be the creation of combinations of cloned human antibodies that can effectively counteract BoNT/A and BoNT/B exposure.